Arbor Scientific http://www.arborsci.com/ which is linked from our
supplies page, offers the following:
"Product ID: P2-7150-02 Category: No Category Concave Mirror
Description: These extra-large parabolic mirrors make demonstrating
optical principles a snap. Each silver-backed, non-aluminized mirror
measures 24" in diameter and is supplied with an aluminum frame, mounting
bracket, and base. Price: $100.00"
Of course, that's larger than 40cm and, although smaller ones are
available, you are right that the 40cm size does not seem to be offered
anymore. You might consider getting a smaller mirror and doing the
experiment with something smaller than a light bulb. A parabolic mirror
is a concave mirror designed so that it won't produce spherical
aberration. For an explanation see:
http://www.physicsclassroom.com/Class/refln/U13L3g.html There are many
other web sites that discuss this as well.

Nina Thayer
Snacktalk Moderator

> I'm looking for a mirror for the "touch the spring" project. I can't
> find one that's as big as 40 cm. I checked all of the links that you
> have for materials. The largest is 10 cm. I did find a large parabolic
> mirror, but I think that when you say concave mirror you mean a
> spherical concave mirror. Is that correct? Am I looking for a
> spherical mirror? Actually looking in the book, I see a reference to
> both spherical and parabolic mirrors. Will both work?
>
>